This invention relates to methods and compositions for reducing the viscosity of circulating media used in primary fractionators of ethylene plants. In an ethylene plant, hot cracked gases from furnaces need to be cooled down (quenched) for further processing and fractionation. This cooling process takes place in the quench system, which in the case of liquid and mixed feed crackers, consists of a series of transfer line exchangers (TLX or TLE), primary fractionator or quench oil tower, quench oil loop, and auxiliary equipment, (i.e. filters, optional fuel oil stripper, etc.). After initial cooling (quenching) in the TLX, cracked products are fed to the distillation tower (primary fractionator or quench oil tower), which separates light products to the top (pyrolysis gasoline) and heavier hydrocarbons to the bottoms. A portion of the bottom product is circulated via the quench oil loop hack to the TLX as the quenching medium. The immense heat recovered through the quench oil system is used to produce dilution steam, which in turn is returned to the cracking furnaces aiding in overall heat recovery.
High temperatures and long residence times in the circulation (quench oil) loop are conductive to the heavy molecules produced in the cracking furnaces to agglomerate into large polynuclear aromatic species, often referred to as tars. Tars increase the viscosity of the circulating media thus increasing the potential for fouling in the tower and in turn negatively affecting heat recovery and proper fractionation.
As the quenching material's viscosity increases, its heat transfer coefficient drops. Over time this can result in severely reduced heat recovery, less steam production in dilution steam systems, and significant costs of import steam required for cracking. Additionally, as the media becomes harder to pump, it works less effectively, or needs to be supplemented with imported flux oil. In some cases the quenching media becomes so ineffectual that at least some portions of the plants must be operated at temperatures beyond their design limitations. All this results in significant costs and problems with maintenance and product quality.
Prior art methods of mitigating viscosity increases in quench media involve adding specially formulated antifoulants that prevent heavy tars from aggregating and depositing, thereby improving the tar's flow characteristics. U.S. Pat. No. 5,985,940 describes the use of phenol-formaldehyde resins to control viscosity in quenching media. The prior art methods, however, lose effectiveness when significant amounts of residual reactive monomers are present in the effluent of the cracking process. Recent changes in industry practice involve using different feedstocks which result in large amounts of residual reactive monomers which impair the effectiveness of these antifoulants. This is causing unpredictable changes in the mechanisms and characteristics of the quenching media and is once again resulting in significant problems in cost, product quality and maintenance. Thus there is clear need and utility for methods and compositions useful in reducing the impact of tar on the circulating media in primary fractionator quench loops.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.